Electrical steel sheets include silicon (Si) and have good magnetic characteristics. Such electrical steel sheets are used to form the cores of rotary machines such as motors and generators, and in a core manufacturing process, circular electrical steel sheets having circular center openings are sequentially stacked and welded together.
Electrical steel sheets for forming such laminated cores are coated with an insulation material to reduce eddy current loss in laminated cores, and organic-inorganic composite coating materials in which organic and inorganic substances are combined are widely used for coating such electrical steel sheets, due to punching-process characteristics thereof.
Such an electrical steel sheet laminated core is processed by a mechanical joining method such as clinching or a welding method so as to prevent separation of stacked electrical steel sheets when the laminated core is carried or copper wires are inserted into the laminated core during a manufacturing process. Particularly, when small laminated cores for home appliances or electric vehicles are welded, a tungsten inert gas (TIG) welding method not using a filler metal is used.
However, such a mechanical joining method or welding method results in magnetic loss due to mechanical/thermal strain and core loss due to short circuits between stacked electrical steel sheets.
Although Non-patent Document 1 states that loss caused by short circuits is lower in a clinching method than in a welding method, the clinching method does not guarantee sufficient joining force for large (tall) laminated cores used in high-power/high-efficiency motors of hybrid vehicles or electric vehicles. Therefore, the welding method is still used.
It has been reported that the performance deterioration of laminated cores caused by welding is proportional to the widths and depths of welds affected by the amount of input welding heat or the number of welding points. Therefore, the amount of input welding heat or the number of welding points has to be minimized to reduce performance deterioration. However, there is a limit to reducing the amount of input welding heat or the number of welding points, in the case that large cores are used, to increase the output power of products such as motors. In addition, while plasma or laser welding has been proposed to reduce the amount of input welding heat, such a method has limitations in securely joining electrical steel sheets.
An organic-inorganic composite material coated on electrical steel sheets for insulation may generate gas while decomposing during a welding process, and when molten metal solidifies, such gas may permeate into the metal to cause porous defects in a weld and thus to decrease a joining force.